The present invention relates to a getter device in which a getter material containing a barium-aluminum alloy powder and a nickel powder is filled in an open annular metal getter container for evaporation of barium upon heating.
A getter device, in an evacuated and sealed envelope, is generally heated by methods such as high frequency induction heating to form a getter film of barium on the inner wall of the evacuated envelope. Before such a procedure, the getter device may be exposed to heat which is undesirable. This applies, for example, to the manufacturing process of a picture tube as disclosed in the specification of Japanese Patent Publication No. 49-12,031. According to this specification, a getter device is mounted inside a picture tube composed of a panel part and a funnel part which are not yet sealed with frit glass. After heating at about 400.degree.-450.degree. C. for 1 hour in air, the panel part and the funnel part are sealed with frit glass.
A general getter material containing a mixed powder of BaAl.sub.4 powder and Ni powder (weight mixing ratio: about 1:1) generates mainly nickel oxide (NiO) by oxidation when heated at over about 350.degree. C. in air for a long period of time. When NiO is present in the getter device, NiO and BaAl.sub.4 react rapidly at high temperatures. When evaporating barium by heating the getter device (to be referred to as a getter flash hereinafter for brevity), this results in an explosive release of barium. When NiO is produced in large amounts, the metal container melts and explosively scatters with the getter material. This kind of explosive scattering must be completely avoided in, for example, a color cathode ray tube since it tends to cause degradation in withstanding voltage. Due to this, a getter device which will not cause problems at high temperatures in air has been desired.
In order to accomplish this, a getter device coated with an organic silane is disclosed in Japanese Patent Disclosure No. 52-84,960, and a getter device coated with silicon oxide is disclosed in Japanese Patent Disclosure No. 52-139,355.
Japanese Patent Disclosure No. 52-84,960 teaches that a getter device coated with an organic silane such as polysiloxane containing alkyl, allyl, aralkyl, alkalyl or hydrogen is capable of withstanding heating at 420.degree. C. for one hour for evaporation of barium, without causing explosive scattering.
However, a getter device coated with such an organic silane presents the defects to be described below during use. A getter device of this type mainly produces a great amount of hydrocarbon-based gas during the getter flash. The produced gas is not easily adsorbed in the getter film, so that the pressure inside the tube is left at about 10.sup.-3 Torr after the getter flash.
As is well known, such a great amount of residual gas is ionized, accelerates and collides with the cathode or the anode applied with a high voltage such as in a cathode ray tube. It is well conceivable that, due to this so-called sputtering effect, part of the electron emissive material on the cathode scatters to other places, significantly degrading the withstanding voltage.
Japanese Patent Disclosure No. 52-139,355 teaches that a getter device coated with a silicon oxide film is capable of withstanding heating at 450.degree. C. for one hour in air, and that such a silicon oxide film is obtainable by immersing the getter device in an ethyl silicate solution prepared by hydrolysis of a composition consisting of, for example, methanol, deionized water and nitric acid, and heating the remaining silicate at 120.degree. C. in a vacuum. Such a getter device shows significant resistance to oxidation at high temperatures. When a getter device which does not have such a protective film is heated at 450.degree. C. for one hour in air and undergoes a getter flash in a vacuum, explosive scattering occurs. However, with a getter device whose surface is coated with a silicon oxide film as described above, when it is heated in air and undergoes a getter flash in a vacuum, the degree of the explosive scattering becomes slight, and only a small amount of the sintered getter material is removed or peeled off to the outside of the chamber. However, even slight explosive scattering and peel-off of the getter material should be avoided completely in an electron tube such as a cathode ray tube, because those phenomena significantly degrade the withstanding voltage of the electron tube. The explosive scattering tends to cause adherance of the scattered particles at undesirable places of the tube, resulting in degradation of the withstanding voltage and frequently resulting in short-circuiting. The peel-off of the getter material tends to cause formation of a barium film at undesirable places of the tube, and this results in degradation of the withstanding voltage. One of the possible reasons for the explosive scattering is the oxidation of nickel in the getter material, although this may only result in a slight amount of explosive scattering. The surface of the getter device coated with a silicon oxide film as described hereinbefore was observed with an electron microscope and it was found that the silicon oxide film consisted of a porous structure. It is thus considered that oxygen is supplied to the getter device through these small holes and part of the getter material is oxidized.